Monday, 11 August 2003: 12:15 PM
Considerations for the Detection of Low Lying Winter Weather in the Canadian Weather Radar Network
Low reflectivity and low lying snow is a prevalent feature of winter weather in Canada and often escape detection by radar due to overshooting. Some of the snow is topped as low as one kilometer about the ground. Over the years, trials at the King City radar combined with FFT Doppler ground clutter filtering indicated the potential usefulness for greater radar coverage and detectability with zero degree elevation angle scans. Some of the radars in Canada are situated on mountain tops and negative elevation angles were tested there as well. An analysis was conducted using digital elevation data for each radar in the Canadian Weather Radar Network to determine the best elevation angle for the detection of low lying weather. The analysis also considered population coverage issues, partial and full beam blockage, increased ground clutter and it was called the Lowest Local Application Angle (LOLAA). Another angle for best long range coverage was specified as well (Best Angle for Long Distance – BALD). It was recognized that quantitative reflectivity estimates and precipitation estimates using these scans would be compromised due to violation of the beam filling assumption or over correction by the ground clutter filters. The analysis indicated that angles as low as -0.6o were needed. In one mountain site, only a very narrow but populated valley would be covered by LOLAA. LOLAA and BALD were implemented operationally in the winter of 2002/2003 across the Canadian Weather Radar Network. It proved highly successful and well received by the forecast community. Weather was detected by radar in locales that were “blind” in the past and weather warnings were issued that previously would not have been previously made. Evaluation and assessment of observations and simulations indicate that some of the LOLAA angles may have been slightly low, sea clutter was more prevalent and clutter filtering was too aggressive.